docs: def SA_guppy_head(D, a):
      r'''Calculates the surface area of a guppy head according to [1]_.
      Some work was involved in combining formulas for the ellipse of the head,
      and the conic section on the sides.
  
      .. math::
          SA = \frac{\pi  D}{4}\sqrt{D^2 + a^2} + \frac{\pi D}{2}a
  
      Parameters
      ----------
      D : float
          Diameter of the main cylindrical section, [m]
      a : float
          Distance the conical head extends, [m]
  
      Returns
      -------
      SA : float
          Surface area [m^2]
  
      Examples
      --------
      >>> SA_guppy_head(2, 1)
      6.654000019110157
  
      References
      ----------
      .. [1] Weisstein, Eric W. "Cone." Text. Accessed March 14, 2016.
         http://mathworld.wolfram.com/Cone.html.'''
      return pi*D/4*(a**2 + D**2)**0.5 + pi*D/2*a